To clarify the origin of the superior field emission characteristics of carbon-coated emitters, the authors investigated the field enhancement and the work function of model systems calculated by numerical simulations. They propose that the field enhancement is due to the triple junctions, which are distributed on the surface of the carbon film consisting of sp3 (diamond-like) insulating and sp2 (graphite-like) conducting nanometer-sized grains. The electric field around the triple junction is one order of magnitude higher than at other places. Based on ab initio density functional theory calculations, the authors found that (1) the work functions of diamond and graphite dramatically decrease down to 3–3.6 eV upon hydrogen termination, and (2) the effective work functions of these models decrease to 2–2.5 eV by applying an external electric field of 2.57 × 107 V/cm. They also estimated the field emission current from the potential distribution and the local density of states under the external electric field applied. As a result, the authors found that hydrogen termination significantly increases the field emission current. The results suggest that the triple junction and hydrogen termination are promising candidates as the mechanism of improving the emission of the carbon-coated emitters.